The invention is based on a method for determining the direction of the earth magnetic field, the north direction and the travelling direction of a vehicle by means of a magnetometer fixedly mounted on the vehicle to measure components of a magnetic field effective at the magnetometer and by means of an evaluating circuit which cyclically evaluates and stores the measured components as electric measurement values defining parameters of an aliphatic locus diagram of the magnetic field inclusive of the magnitude and direction of hard-magnetic and soft-magnetic interfering fields, and calculates the direction of the earth magnetic field with respect to the travelling direction.
From DE No. 3,509,548 corresponding to the U.S. Pat. No. 4,738,031, the entire disclosure of which is incorporated herein by reference, a method for determining the travelling direction of a vehicle having an electronic compass is known in which, by means of a magnetometer mounted in the vehicle, the magnetic field effective in the vehicle is measured and stored in the form of an elliptical locus diagram in an evaluating circuit and is processed for calculating the earth field direction with reference to the driving direction. A calibration travel is used to determine as parameters of the elliptical locus diagram of the magnetic field the displacement of the locus diagram from the origin of the coordinates, the rotation and the minor and the major semi-axis of the elliptical locus diagram. Furthermore, it is there known to perform, if the cyclically measured magnetic field deviates from the elliptical locus diagram by a predeterminable amount, a recalibration of the locus diagram immediately or if the deviation is repeated several times.
However, such a dynamic recalibration for compensating interfering field changes in the motor vehicle must be conducted in such a manner that interfering fields or interfering field changes occurring for a short time are suppressed, if necessary, whereas longer-duration or slow interfering field changes in the vehicle must be compensated. In this connection, it must be taken into consideration that the interfering fields existing in the vehicle can change in dependence on time and that the field strength of the earth magnetic field is subject to strong fluctuations which are essentially location-dependent. In the case of the interfering fields, this results in a direction-independent interfering field vector as a hard-magnetic component which is changed, in particular, by electric currents flowing in the vehicle. The connecting and disconnecting of individual loads produces abrupt magnetic field disturbances. These can cause remagnetizations in the vehicle steel which result in relatively slow changes of the direction-independent interfering field vector after days or weeks. The direction-dependent interfering field vector, as a soft-magnetic component of the interfering field in the vehicle, changes both due to the change in intensity of the earth field with drives in the north-south direction and also with a remagnetization in the vehicle steel. These, too, are relatively slow interfering field changes.
The present invention has the object of including these interfering field changes with long-term effect step-by-step in the recalibration.